Marine seismic surveying



Feb- 14, 1967 w. B. HUCKABAY ETAL 3,304,533

MARINE SEISMIC SURVEYING File-d Dec. 4, 1964 e sheets-sheet 1 ATTO'Q/VEYS Feb. 14, 1967 Filed Dec. 4, 1964 W. B. HUCKABAY ETAL.

MARINE SEI SMIC SURVEYING 6 Sheets-Sheet 2 IF' L E E Feb. 14, 1967 FiledDec. 4, 1964 W. B. HUCKABAY ETAL MARINE SEI SMIC SURVEYING 6Sheets-Sheet 3 D a INVENTORS a WML/AM 6, Hua/AMY; I/V/LL/AM H, PAQA/f-eFeb. 14, 1967 w. B. HUcKABAY ETAL 3,304,533

MARINE SEI SMIC SURVEYING 6 Sheets-Sheet 4 Filed Dec. 4, 1964 mun QTL"@k m-IL WFL .m WFL WE NFL Feb. 14, 1967 Filed Dec. 4. 1964 w. B.HUCKABAY ETAL 3,304,533

MARINE SEISMIC SURVEYING 6 Sheets-Sheet 5 A fm2/v5 Ys Feb- 14, 1967 w.B. HUCKABAY ETAL 3,304,533

MARINE SEI SMIC SURVEYING United States Patent iitice 3,3%,533 PatentedFeb. 14, 1967 3,304,533 MARINE SEISMC SURVEYING William IE. Huckabay,Dalias, and William H. Pariser, Richardson, Tex., assignors to RayfiexExploration Company, Dallas, Tex., a corporation of Texas Filed Dec. 4,1964, Ser. No. 416,085 2 Claims. (Cl. 340-7) This inventionrelatesgenerally, as indicated, to irnprovements in the art ofconducting seismic surveys of water-covered areas, and moreparticularly, but not by way of limitation, relates to an improvedmethod and apparatus for conducting marine seismic surveys utilizing anelectrical arc in the water as the energy source.

This invention is an improvement upon the subject matter disclosed inthe copending application of Huckabay, et al., Serial No. 232,202, nowPatent No. 3,251,027, tiled on October 22, 1962, and entitled SeismicExploration.

As is well known in the seismic art, it is common practice to conductseismic surveys of water-covered areas. In such surveys, a pressure waveis created in the water which travels downwardly and becomes a seismicwave as it enters the earth. The seismic waves reflected from subsurfaceinterfaces travel back upwardly and are reconverted into pressure wavesin the water. These latter pressure waves are in turn detected bytransducers known as hydrophones, whereby information relating to thedepth and configuration of subsurface strata may be determined.

Various types of energy sources have been utilized to create pressurewaves in a body of water, such as charges of explosive, gas guns, etc.However, one of the present day most popular energy sources is anelectrical source utilizing spaced electrodes in the water which createpressure waves by forming an arc between the electrodes. An electricalenergy source of this type is highly useful in being particularly suitedfor rather rapid firing when a continuous seismic survey is beingconducted, and such devices have been found to provide an appreciableservice life. On the other hand, however, such electrical devices haveone shortcoming in common with all other known energy sources as theyare presently utilized. The pressure wave and, hence, the resultingseismic wave, resulting from the tiring of an electrical energy sourcein a body of water `contains a substantial range of frequencies; whereasit is usually desirable to have a rather limited range of frequencies inthe seismic wave transmitted into the earth. As a result, a substantialportion of the energy generated by the source is not efficientlyutilized. Also, the single firing of an electrical energy source of thetype under discussion normally contains a predominant number offrequencies higher than those desired for seismic exploration.

The present invention contemplates a novel system of marine seismicsurveying wherein two electrodes are placed in the body of Water and aseries of potentials are imposed across the electrodes in rapidsuccession to, in effect, build up the pressure wave created in thewater and control at least the predominant frequency of the resultingseismic wave. In other words, the electrodes are sequentially fired in amanner to shape the resulting seismic wave. In one embodiment, theelectrodes are fired at varying potentials to obtain a resulting seismiclwave resembling a sine wave of the desired frequency and in a secondembodiment, the electrodes are fired at substantially equal potentialsto obtain a resulting seismic wave more in the nature of a square waveof the desired frequency content.

The present invention also contemplates a novel firing circuit for apair of electrodes immersed in a body of water wherein the potentialsimposed across the electrodes may be easily controlled as to amplitudeand rate in order to obtain a resulting seismic wave of the desiredamplitude.

One object of this invention is to improve the efficiency of a marineseismic energy source utilizing a pair of electrodes immersed in a bodyof water.

Another object of this invention is to provide more control over thefrequencies contained in seismic waves utilized in marine seismicsurveying.

A further object of this invention is to improve the signal-to-noiseratio in marine seismic surveying operations.

A still further object of this invention is to shape the seismic wavesutilized in marine seismic surveying and yet form such waves withsuilicient rapidity to allowl a continuous type of survey. A v

Other objects and advantages of the invention will be evident from thefollowing detailed description when read in conjunction with theaccompanying drawings which illustrate the invention.

In the drawings:

FIG. 1 is a curve illustrating a typical amplitude spectrum of apressure wave generated in a body of water by the creation of an arcbetween two electrodes immersed in a body of water.

FIG. 2 is a curve showing a typical frequency analysis of a pressurewave such as that illustrated in FIG. 1.

FIG. 3 is a curve illustrating one type of pressure wave which can beobtained by use of the present invention.

FIG. 4 is an alternate pressure wave shape which can be obtained by thepresent invention.

FIG. 5 is a schematic illustration of a portion of the apparatusutilized in a marine seismic survey in accordance with this invention.

FIG. 6 is a block diagram of the circuit utilized in imposing a seriesof potentials across electrodes in rapid succession to create thepressure waves illustrated in FIGS. 3 and 4.

FIG. 7A is one portion of a wiring diagram of the firing rate controlcircuit of this invention.

FIG. 7B is a continuation of the wiring diagram illus trated in FIG. 7A.

FIG. 8 is a block diagram of an alternative embodiment of the arcingcontrol circuits and electrodes as shown in FIG. 6.

As indicated above, the shape of a pressure wave in a body of watercreated by an arc passing between a pair of electrodes immersed in abody of water is illustrated in FIG. l. It will be noted that the waverises somewhat like a sine wave and then decreases in a similar mannerto below zero. At this time the wave becomes erratic in a negativedirection as a result of what is known as cavitation in the waterresulting from a release of pressure in the water. Following arelatively short period of cavitation, about fourteen milliseconds, thepressure wave rises sharply to a substantial amplitude and then fallsoff sharply to zero.

A frequency analysis of a single arc pressure wave as shown in FIG. 1 isillustrated by the solid-line curve in FIG. 2. It will lbe noted thatthe greatest percentage of the pressure wave is constituted by the lowerfrequencies, with a proportionate decrease as the frequency increases.This illustrates very well the desirability of an arc generated pressurewave since the frequencies at about fifty to one hundred cycles havebeen found to be the most useful in seismic work. The frequency analysesof FIG. 2 have been taken with respect to the pressure wave created inthe water, but it has been found that a resulting seismic wave will haveat least approximately the same frequency content.

The pressure wave generated by the firing of a single arc yields aneiciency of thirty to forty percent of de* sirable frequencies. This isa great advantage in itself over other prior art methods; however, themultiple 3,304 ,ess

J ig technique of this invention has been found upon lysis to furtheraccentuate the desirable frequencies the pressure wave. The broken-linecurve of FIG. pproximates a frequency analysis of the pressure wave atedby six sequential arcs. An efficiency as high as l to sixty percent ofthe pressure wave has been found the low frequencies of fifty to onehundred cycles, i1 complementary decrease of higher frequencies andaneous disturbance. n accordance with the present invention, a series ofssure waves are created in the body of water in rapid cession asillustrated, for example, by the curves l() )ugh 17 illustrated in dashlines in FG. 3. The ves through 17 represent the initial portion of assure wave of the type illustrated in FIG. l and are ated in sucientlyrapid succession that the waves flap and reinforce one another. As aresult, the il pressure wave created in the water -will have thefiguration illustrated by the solid curve 2u when the Iividual pressurewaves iti through i7 are varied in Jlitude as shown in FIG. 3. .t willbe noted in FIG. iat the individual pressure waves l@ through 17 are 1ergradually increased in amplitude and then gradly decreased in amplitude,such that the resulting elope, the solid curve 2t), resembles thepositive porl of one cycle of a sine wave. Further, it will be ervedthat the width of the wave 20' will be equal to -half of the wave lengthof the resulting pressure Ie and, hence, approximately7 one-half thewave length he seismic wave resulting from the pressure wave. It bereadily understood by those skilled in the art that controlling therelative amplitudes of the individual ssure waves 10' through 17, theresulting pressure fe 29 may have substantially any desired predominantiuency content, such as from fifty to one hundred les as is normallypreferred in seismic surveying. n lieu of an approximate sine waveconfiguration, the ividual pressure -wa'ves created may be provided withstantially equal amplitudes as illustrated by the waves through 2S inFIG. 4. In this case, the individual ssure waves 22 through 28 areprovided in rapid suc- ;ion and in overlapping relation to provide anoverail elope (illustrated by the solid line wave 30) subitially in theform of a square wave. The wave 30 is, :ourse, the resulting pressurewave. This wave shape ltilized when it is desired to provide more highfrencies in the seismic wave than are obtained with a es of rapidlysuccessive pressure Waves of the type wn in FIG. 3. n conducting amarine seismic survey in accordance 1 this invention, a vessel 32 (seeFIG. 5) is steered ig a predetermined course through a -body of wateroverlying the subsurface structure to be surveyed. A able towing cableextends from an electrical power ply and associated circuits 3S on thevessel 32 into the er 34- for towing one electrode 4i@ forming a cathodewill be described) and a plurality of associated elecles 42 forminganodes. The electrodes 40 and 42 are ed through the water 34 at thedesired depth lby use of litable oat 44 in a conventional manner.llectric arcs are passed between the cathode 4u and t1 of the anodes 42,in sequence, as will be described re in detail below, to produce thepressure waves in water 34. Each pressure wave travels outwardly indirections from the electrodes, including downwardls indicated by thearrows in FIG. 5. As each presi wave reaches the sea-bottom or surface45 of the h, the pressure wave is converted into a seismic wave chtravels on downwardly through the substructure. h interface 48 of thesubstructure causes a reflection a portion of the seismic energy asindicated by the )ws at each of these interfaces. The reflected seismicyes travel upwardly and are converted into pressure yes in the water 34which in turn travel on upwardiy. :se latter pressure waves representingthe reflected seismic waves are detected by a plurality of hydrophones59 which convert the pressure waves to electrical signals. Electricalsignals are in turn transmitted through another towing cable 52 tosuitable recording equipment 54 0n the vessel 32.

As shown in FlG. 6, the electrode 40 forming a cathode is preferably inthe form of a tubular member constructed of any suitable :conductingmetal, and each of the electrodes 42 is preferably the exposed end of aconventional shielded cable of the type commonly used in marine seismicwork. Although not shown in the drawings, the electrodes 42 are suitablysecured and insulated from the electrode 40 with the electrodes 42 beingspaced equal distances from the electrode 40, such that when a givenpotential is imposed across the electrode 40 and anyone of theelectrodes 42, substantially the same amplitude of pressure wave willthe created in the body of water in which the electrodes are immersed.

In order to provide -a series of rapidly successive pres- 20 Wsure wavesas previously described in connection with FIGS. 3 and 4, we provide aplurality of D.C. power supplies 56, each of which has a positiveterminal 58 and a negative terminal 66. Each power supply 56 is providedto impose the desired potential across a high voltage capacitor 62,connected lacross the terminals 58 and 60, which can be rapidlydischarged for creation of the arc between the electrode 40 and one ofthe electrodes 42, as will be described. All of the negative terminals60 of the power supplies S5 are connected in parallel to the cathode 4?,and each of the anodes 42 is connected separately to a positive terminal53 of its respective power supply Se through a switch 6ft. It will,thus, be apparent that when one of the capacitors 62 is charged, and therespective switch 64 is closed, the capacitor 62 will discharge throughthe respective anode 42 and the cathode 4G, and hence through the bodyof water, to provide an arc in the body of water which generates theresulting pressure w-ave. It wili also be apparent that the amplitude ofthe pressure wave will be dependent upon the charge stored in therespective capacitor 62, such that the ampiitudes of the pressure wavesproduced by the various power supplies 56 may be controlled bycontrolling the potential produced by the power supplies.

In a preferred embodiment, each of the switches 64 is in the form of anionic switch which is closed by the imposition of a potential on afiring electrode 66 extending into an ionic chamber, as is well known inthe art. The firing electrode 66 of each ionic switch 64 is connected toa tiring rate control circuit 68 shown as a block in FIG. 6, but whichyis shown in detail in FIGS. 7A and 7B and will be explained in detailbeiow. At this point it is sufcient to say that the ring rate controlcircuit 68 is enabled at input 70 by a trigger pulse from a suitablegenerator to sequentially energize and close the ionic switches 64 at arapid rate, thus providing a series of successive pressure waves in thebody of water in which the electrodes 40 and 42 are immersed. The totaltiring time of all of the ionic switches 64 in each sequence ofoperation will, of course, determine the wave length of the resultingpressure wave. It will also be apparent that the number of anodes 42 andassociated power supplies will be a controlling factor on the waveshapes possible for the resulting pressure wave. In one experimental useof the invention, we have utilized nine anodes 42 and nine associatedpower supplies 56 in a marine seismic surveying operation.

As indicated above, the ring rate control circuit is shown in detail inFIGS. 7A and 7B. Referring rst to FIG. 7A, it will be observed that thecircuit 68 includes a decade counter tube 72, having a single anode anda plurality of cathodes to provide a succession of nine operations eachtime the tube is fired. We have used a Sylvania 6910 cold cathode decadecounter tube.

The decade counter tube 72 is a gas-filled tube which has an anode, tencathodes `and twenty guide electrodes.

E' J The anode is placed in the center surrounded by the ring of tencathodes, with two guide electrodes placed between each two adjacentcathodes. The standard schematic representation, which is used in FIG.7A, only shows two guides but it should be understood that each ofguides .G-11 and G12 is connected in parallel with nine other guides,thus making a total of twenty, and placing one guide electrode of eachparallel network between each two adjacent cathodes. It is by means ofthe guide electrodes that the glow or energization of each cathode isenabled in =a predetermined sequence. By pulsing the guides alternatelywith a more negative potential, the glow can be shifted from one cathodeto the next around the entire ring of cathodes. This is well known tothose skilled in the yart and it is standard operation for the Sylvania6910 counter tube.

Unless provision is made, the counter tube 72 will count around itscircle of cathodes continually. Hence, the No. cathode is reserved inthis application for use as a start and reset means for controlling thecount or rotational 'energization of the cathodes. This is done asfollows: when the tube 72 is in a non-counting condition, that is, theperiod in the pressure wave tiring cycle when no output is desired, theNo. 10 cathode is held at a sufliciently negative potential of aboutminus sixty voits which causes the glow to remain on that cathode untilshifted. The shift will then take place by raising the negativepotential to a less negative, minus five volts so that the periodicvoltages to the guides G-ll and G-12 can take over and shift the glowaround the tube. This shift is initiated by a trigger pulse from thetiming means at the recorder (not shown) of the surveying system thussynchronizing the pressure wave firing means with the recorder.

More detailed description of the counter tube operation will follow, butit should be understood that although the pulse volt-ages from thesequencing generator (stages 100, 102 and 104 as will be described) willbe present at the guides of counter tube 72 at all times, the glow-shiftcan take place only during the short firing interval when the bi-stablemultivibrator 74 is shifted to remove the high negative bias from theNo. 10 cathode of tube 72.

The enabling stage which removes the high negative bias on the countercathode is a dual triode 14. This stage functions as a bi-stablemultivibrator with halves 74A and 74B conducting in opposition to oneanother. The cathodes are grounded and each grid is controlled by asimilar bias network. Each of the grids is biased from the regulated-105 volts. Diodes 76 and '78, one at each grid input, are polarized toallow application to the grids of a high negative switch pulse (to bedescribed). At the same time, the diodes serve to block any flow of gridcurrent to the stabilizing bias supply of plus sixty to eighty Volts.

A functional description of the bi-stable multivibrator will betterillustrate the operation of the circuit. Assuming a condition when nopressure wave firing is taking place, tube 74A is conducting and 74B iscut off. When 74A is conducting the sharply reduced plate voltage at theplate of 74A causes a more negative voltage to appear across plate-gridconnector 80 to the grid of 74B. This lowered grid voltage at 74B tendsto maintain the tube cut off, and therefore with maximum plate voltage;this, in turn, couples a raised voltage via plate-grid connector 82 backacross to the grid of tube 74A, maintaining it in conduction. Hence, thestage 74 is in one stable state of operation and will remain there untiltriggered into its other state.

While stage 74 is in this condition no firing is taking place as thecounter tube 72 is disabled as follows. When triode 74A is conducting, areduced voltage is present across connector 80 which controls the gridof 74B as Stated above. This reduced voltage is also connected to thegrid of triode 84 through neon glow tubes 86, S7 and 88 to the gridbiasing network 90. Tube 84 operates be tween the regulated -105 voltsand ground; hence, th cathode-grid bias is derived from the 105 voltsuppl with the plate connected directly to ground. Neon tube 86, 87 and88 serve to drop the voltage from connecto by sixty volts each, or alSO-volt total, thus lowerin the range of voltage variation to valuescompatible wit proper grid control of tube 84.

It is seen then that conduction of tube 74A will re fiect a reduced gridvoltage at grid bias point thu maintaining minimum conduction throughthe ltube 84 ln this condition very little current is drawn through thcathode resistor 92, causing less voltage drop and, there fore, a morenegative voltage across connection 94 to th No. l0 cathode of countertube 72. This voltage is'o the order of minus sixty volts and issufficiently negativ with respect to the remainder of the cathodes andguide to cause the glow to dwell on the No. 10 cathode. Sine there is noglow rotation (sequential cathode enabling there can be no sequentialoutput from the counter tub 72, and no ring of the arcing members.

At the predetermined firing time, a trigger pulse is ap plied at input96 which is sufficiently negative to cut ot tube 74A. This trigger pulseis on the order of -100 volts and is generally obtained from time-basecircuiti` associated with the recorder equipment. This pulse i appliedthrough diode 76 to the grid of tube 74A, thu ceasing its conduction andIallowing a sharp voltage ris at the plate of tube 74A. The rise involtage is seen a the grid of 74B, tiring it into conduction, thusdecreasin the plate voltage of 74B and, in turn, the grid voltage o 74Aso that it is held at cut olf. Multivibrator 74 is no\ switched to itssecond stable state; that is, the state dui ing which ring is enabled."te

With 74B conducting and 74A cut olf, the increase i voltage appearing atthe plate of 74A is also conducte` to the grid bias network 90 of tube84 where it raises th voltage and increases conduction through tube 84.Thi increased current through the cathode resistor 92 of tub S4 bringsabout a substantial increase in the voltage dro thereacross; hence,connection 94 places a more positiv bias (about minus ve volts) oncathode No. l0 o counter tube '72. This is sufficient reduction innegativ biasing of cathode No. l0 to allow the glow or energiza tion tobe rotated to other cathode elements in the counte tube 72, since eachof cathodes No. 1 to No. 9 is biase at a more negative minus twelvevolts by supply lead 9( A second negative pulse is applied to the gridof triod '74B for reversing the operation or order of stability omultivibrator 74. This will be described later in conne( tion withdisabling of the firing sequence.

Glow rotation having been enabled, in order to pass th glow from onecathode element to the next in counte tube 72, it is necessary to supplyalternate negative puls ing to the guides G-ll and G-12 situated betweeneac of the cathodes. The guides are biased slightly more pos: tive thanthe cathodes, so that preferentially the glow will rest on a cathoderather than one of the guide ele ments. The preference of the glow torest on the prope cathode, even though a negative transfer pulse isapplic to ten guide elements simultaneously, is because the prim inginfluence of the discharge moves the glow to that cati ode which isclosest to the discharge. The proper ad justment of anode potentiometer97 insures that th anode-cathode potential of counter 72 is at the propevalue for maintaining glow-on only a single main cathode The alternatingnegative pulse voltages to the G-i and G-12 guides are supplied by theaudio oscilaltor 10 and amplier stages comprising dual triodes 102 andlib The timing source 100 is a standard audio oscillato capable offrequency yand amplitude adjustment. Th frequency chosen will bedependent upon the wave-shap duration which an operator may decide uponusing; sine: the wave length of the input timing signal will contre thesequence shift of the counter and, therefore, th

ount of time between each of the arcs as they are uentially enabled.Thus, the length of the resultant ssure wave can be controlled for bestutilization of equipment as might be dictated by the subsurface ter- 1,water temperature, and other factors which might The output ofoscillator 1li@ is a sine wave signal hava peak-to-peak amplitude of tenvolts. This signal lpplied through coupling capacitor 136 to the grid ofoverdriven A.C. amplier 102A operating between 05 volts 'and ground andutilizingr grid leak bias. The plied signal at the plate of tiZA isconducted to two owing stages. First, it is applied through couplingacitor H4 to the grid of driver triode 104A where the ial controlsVconduction from the plate circuit of tube LA to the guides G-lll of thedecade counter tube 72.. ond, the same yamplified signal (from the plateof LA) is applied to the grid of triode 1MB where it is plified andreversed in phase for conduction through pling capacitor 116 to the gridof driver triode 1MB. ver 104B controls conduction to the guides G-lZ ofcounter tube 72 in the same manner as driver HMA, always in the reversedphase; i.e., either the conduc- 1 or non-conduction state.

l switch S which parallels capacitors il@ and i12 h capacitors lid andH6 respectively, provides addiial coupling capacity when the operatingmode chosen i lower frequency range. The diode lf3 is provided dischargeof coupling capacitors 116 and 112 thus iding any grid blocking oftriode TMB. A Zener de 129 provides constant cathode-grid bias of sevennine volts in each of stages 102B, 164A and 1MB. s bias is chosensuchgth'at each of the amplifier stages l be completely cut off duringslightly more than .half its operating cycle. A capacitor 22, of largevalue the order of two microfarads, serves as a decoupling :r betweenthe *105 volt supply and ground; to ich, the plates of 162A and NZB areconnected. The B+ voltage for drivers TMA and 104B is obtained @ugh theguide (G-ll, G42) to anode capacity of the inter tube 72. Thus, as asutliciently positive pulse ears at the grid 124 of amplifier 164A, thetriode s into conduction thereby reducing the voltage drop oss thetriode and causing a negative shift in the potial appearing `at theguides G-l1. in the next half le, triode 1MB is pulsed into conductionand provides egative shift in potential for the guides G-li2. Hence, heach complete cycle of the input audio sine wave, glow is caused torotate about the counter tube 72 means of alternate, negative pulsing ofguides G- l guides G-lZ, once each, and in sequence, so as to .it theenabling glow from one of the cathodes to the `t in succession. ncounter 7?., an output of fifteen volts is developed in h cathoderesistor as its respective cathode is energized ;equence and conductsthrough the tube to the anode. erring now to FG. 7B, each of therespective cathode tages is conducted via leads f2.6-1.3@ to therespective is of a series of trigger tubes, shield-grid thyratrons i444.The type '21321 thyratron has been used with ch success. The outputvoltage from the counter bode, when applied to a grid of one of thethyratrons i-lLi-, is sufiicient to vbring about initiation of the archin the tube thereby firing the thyratrons into conduci. As each of thethyratrons conducts in its turn, it :harges its respective .5 nfd.capacitor T46-154 through espective high voltage coil 156-164 to ground.The h voltage coil chosen is a Delco type coil which autouces a peakvoltage of 10,000 volts (10 kv.) at outputs 5-3174. Each of these lowcurrent, high voltage pulses :onducted to a respective ionic switch(switches 64 of 3. 6) where it serves to ionize the matter in the switchity, thereby completing the circuit between the hightage DC. powersupplies 5'5 and their respective arcing members or anodes d2.. Thus, aseach cathode of counter tube 72 is sequentially enabled, a firing takesplace at a corresponding arcing member in the Water.

Since a ring sequence only requires one complete count or rotationaround the cathodes, the equipment is constructed so that the countertube 72 is disabled after the No. 9 cathode fires its respectivethyratron M14. When thyratron 144 fires (the last of a sequence) anegative pulse is taken from its plate through capacitor 176 and appliedover line 178 to the grid of triode 74B in the bi-stabie multivibrator74. Refer once again to FlG. 7A. During firing, the `stable state iswith 74B conducting and 74A cut off. The negative pulse applied by line178 and diode '73 drives the grid of 74B negative, cutting offconduction and sharply increasing the 4plate voltage at 74B. This platevoltage increase is conducted across connection g2 to the grid of tube74A, causing its conduction; which, in turn, holds 74B at cutoff byrneans of the reduced voltage over the plate-grid bias connection Sti.Thus, the multivibrator 74 is again in its other stable state, thatwhich maintains minus sixty volt bias yon the No. l0 cathode of countertube 72 and restrains it from the counting or glow rotation function.Switches It@ and 182, associated with the multivibrator 74, are normallyopen and can be used for manual start or reset by providing -lS voltbias to the selected grid ofthe multivibrator 74.

The power supply for the firing rate unit is conventional and can be anyof many well known in the art. rl'he requisites of B+ and regulated *105volts are easily provided in standard power supplies. The various biasvoltages called for can each be provided by appropriate voltage dividersof proper polarity.

Referring to FG. 8 there is shown an alternative embodiment of thefiring system as disclosed in FIG. 6. Limitation of space or equipmentmay necessitate the use of the equipment with a single `anode 42 insteadof a plurality of anodes. lf desired. and in some cases it may bepreferred, one anode 42 `of heavy construction can be used for firingeach one of t-he successive high voltage arcs to the cathode 46. Thismerely requires paralleling of each ofthe high voltage cables 65' to onecommon anode 42. The operation of the system remains the same as to thetiring rate lcontrol 68, p-ower Isupplies 56 and ionic switches 64. Thecommon cathode configuration would lbe the `same and the only changerequired is that of combining the high voltage cables 65 by somesuitable connection for enabling multiple arcing from the single anode42.

From the foregoing it will be apparent that the present inventionprovides an electric arc pressure Wave source which is capable ofemitting a more efl'icient pressure wave since it is comprised of agreater percentage of thefrequencies desired in marine seismicsurveywork. The invention is capable of emitting a pressure wave of aparticular, chosen wave-shape, depending upon theV dictates ofconditions incident to the survey. A relatively compact and adjustablefiring 4rate control unit offers the capability of firing multiple highvoltage arcs in rapid sequence to enable a resultant pressure wave ofdesired intensity and frequency distribution. This firing rate controloffers the further advantage that control and sequencing is performedelectronically and in rapid manner7 thus causing no delay which mightdisrupt a continuous survey. Another advantage incident to the firing ofa pressure wave which contains a greater percentage of desiredfrequencies is in the increased ability to detect returned signals andcombat extraneous noise and disturbance.

Changes may be made in the combination and arrangement of elements asheretofore set forth in this specifcation and shown in the drawings, itbeing understood that changes may be made in the embodiments disclosedwithout departing from the spirit and scope of the invention -as definedin the following claims.

We claim: 1. In a system for conducting a seismic survey of subsurfacestructure covered by water:

plural arcing means disposed in the water, plural switch means connectedto the respective arcing means, plural power sources connected to therespective switch means, and a decade Counter eiective to provide aplurality of sequential output pulses, each pulse of which is effectiveto close a switch means and thus enable an are, said decade counterbeing pulse-enabled to deliver sequential output pulses for apredetermined short period, the last pulse of the `sequence effective todisable the counter output. 2. The system as defined in claim 1 whereinthe duration of said coun-ter output is controlled by a bistablemultivibrator, said multivibrator enabling counter outpu of sequentialpulses when pulsed to one sta-ble state by timing pulse, saidmultivibrator disabling said counter out put when pulsed lto the otherstable state by a predetei mined one of the sequential pulses.

References Cited by the Examiner BENJAMIN A. BORCHELT, Pri'naryExaminer.

P. A. SHANLEY, Assistant Examiner.

1. IN A SYSTEM FOR CONDUCTING A SEISMIC SURVEY OF SUBSURFACE STRUCTURECOVERED BY WATER: PLURAL ARCING MEANS DISPOSED IN THE WATER, PLURALSWITCH MEANS CONNECTED TO THE RESPECTIVE ARCING MEANS, PLURAL POWERSOURCES CONNECTED TO THE RESPECTIVE SWITCH MEANS, AND A DECADE COUNTEREFFECTIVE TO PROVIDE A PLURALITY OF